(a) Field of the Invention
The present invention relates to a noise canceller system which removes noise in a television signal and particularly to an image processing system for removing electrical noise disturbance in television signals caused by electromagnetic waves generated by power lines.
(b) Description of Prior Art
In recent years, various devices and systems have been proposed for real time cancelling or reducing noise in a television signal. U.S. Pat. Nos. 4,064,530 and 4,072,984 teach the use of a first order digital filter with a frame delay and a movement detector. A known multiprocessor also teaches the use of the Hadamard transform of frame difference signal for extracting noise. These two known systems, however, require a frame memory, the cost of which is very expensive. Furthermore, the algorithms implemented in these systems, designed for reducing independent white noise are not truly effective for electrical noise which is simply not of the same nature. Faulty isolators on power lines generate sparks of very short duration (a few nanoseconds) radiating electromagnetic waves that are captured by the television signal receiving antenna. The short pulse creates a large disturbance in the receiver that approximates a damped oscillation of a few microseconds duration (the impulse response of the channel). When it is related to the power line frequency of 120 Hz, it produces two bands of thin horizontal speckles drifting across the television screen.
In image processing it is known to use many line masks for detecting those noise speckles. However, in a genuine signal, thin horizontal lines, for example, contour enchancement of alphanumeric characters, are also falsely detected.
In the present invention, a non-linear filter with delayed decision is used as the noise detector. A new signal line detector is also used to inhibit correction of the point erroneously identified as noisy. The signal line detector is simple and effective.